I first read of the C++ “Most Vexing Parse” in the Scott Meyers Effective C++ series. For reference, here’s the code:

double aDouble;
int i(int(aDouble)); // This doesn't do what you think

The commented line actually declares a function i that takes a single integer parameter and returns an integer. Now, today, I saw something similar in F# that made me scratch my head and I think is similar:

The symptom was that my function (here, called func) wasn’t being called. Yet stepping through in the debugger, I could break on the line that called it – yet it wouldn’t step into the function! Of course, by missing one of func’s parameters in the function call, I’d actually declared sum to be a new function taking one parameter (or to use functional terminology, I’d curried a and b).

This is hard to spot when the calling code and function declaration are in separate files and when you’ve added a new parameter to the function and it still compiles but doesn’t do what you expected!